1. Field of the Invention
The present invention relates generally to a method of controlling exposure of a process camera and an apparatus therefor, and more particularly, to a method of controlling halftone photography using sub-exposure in a process camera and an apparatus therefor.
2. Description of the Related Art
Obtaining halftone negative image of an original by using a screen, such as a contact screen and a halftone screen, is called halftone photography. In halftone photography, contrast of the obtained halftone negative image should be made appropriate in accordance with different originals having different densities. The exposure at the halftone photography should be accordingly controlled with high precision.
The halftone negative image is required for the following reasons. In letterpress printing, offset printing, screen printing and the like, continuous tones of an original having contrast such as a photograph are converted to tones represented by the dot areas of the halftone negative image because the continuous tones are difficult to obtain by a printing press or the like. The reason for it is that the printing press basically carries out printing by pressing the surface with ink applied thereto the paper with specific pressure, and therefore does not allow a subtle adjustment of the pressure as a person does.
Referring to FIG. 1, suppose an original having a step-like density distribution as shown in FIG. 1 (b) is exposed to a film through a contact screen by a transmitted light of specific intensity shown in FIG. 1 (a) Hereinafter, "density" of an original is defined as follows throughout this specification and claims attached thereto.
A density Dt of a transparent original (an original through which a light is transmitted) such as a film can be expressed by the following equation: EQU Dt=log (I.sub.o /I.sub.0) . . . (1)
where an intensity of an incident light to the original is represented as I.sub.0 and an intensity of the light after transmitting through the original is represented as
I.sub.t.
It can be seen from the equation (1) that the higher the density Dt is, the smaller the intensity of the transmitted light is. The intensity of the light transmitted through the original having the density distribution as shown in FIG. 1 (b) will be shown accordingly by FIG. 1 (c). In FIG. 1 (c), the light intensity is represented as a width of the arrow. The density Dt defined by equation (1) is referred to as a transmission density throughout the specification and the claims.
On the other hand, printed matters and photographs are not transparent. Therefore, for such an original (reflection original), its density Dr is defined as follows. A standard sample is illuminated with a predetermined light and the intensity Is of the reflected light is measured. The sample for comparison is illuminated with the same light and the intensity Ir of the reflected light is obtained. The density Dr will be expressed by the following equation. EQU Dr=log (Is/IR) . . . (2)
The density Dr defined by equation (2) is referred to as a reflection density throughout the specification and the claims.
For a contact screen, its transmission density Dt is called "screen density". The distribution of screen density of a contact screen in general is shown in FIG. 1 (d).
The light of such amount as shown in FIG. 1 (c) is transmitted through the contact screen having the density distribution as shown in FIG. 1 (d) to reach the film. a film called a lith film is generally used for a photomechanical process. The lith film produces dots when exposed to a light (amount is referred to as an integral exposure amount) of more than a specific amount.
Therefore, when smaller amount of light is transmitted through the original, dots are formed at only the portions where the densities of the contact screen are low. As the light amount transmitted through the original, is increased which is the lith film produces larger dots. In this way, the halftone negative image is obtained having larger dots representing lower density (brighter) portions of the original and smaller dots representing higher density (darker) portions of the original.
A density distribution of a contact screen is peculiar to the contact screen. In addition, a density range of an original which can be reproduced through one exposure by using a contact screen having a predetermined density distribution is defined by a maximum value and a minimum value of the density of the contact screen. Note that a halftone screen, the density distribution thereof is such that light transmitting portions and light shielding portions are alternately arranged, can provide a density distribution equivalent to the one shown in FIG. 1 (d) when the halftone screen is arranged in such a manner that the same is not in contact with the photosensitive material.
Referring to FIG. 2, a difference between the maximum value and the minimum value of the screen density is referred to as "screen range" throughout the specification and the claims. A difference between the maximum value and the minimum value of an original density is referred to as "original range" throughout the specification and the claims.
Original ranges of the originals to be reproduced in practice are different from each other to reproduce of an original having an original range out of a screen range in order to acquire a halftone negative image in a precise tone requires a special controlling method of exposure.
One of the methods for controlling contrast of a halftone negative image in corresponding to various originals in such a halftone photography using a process camera is a sub-exposure method. This method generally includes a main exposure and either or both of a bump exposure or/and a flash exposure.
Referring to FIG. 3, the main exposure is carried out as follows. An original 35 is placed on an original table (not shown) of a process camera. A film 37 and a contact screen 36 put on the original 35 side of the film 37 are supported by a vacuum board (not shown) or the like above the original 35. A lens 11 for forming an image of the original on the film 37 is provided between the original 35 and the film 37. The space between lens holders supporting the lens 11 and the original 37 is light-shielded from the outside by bellows 8, thereby forming a dark space. A flash lamp 14 for use in a flash exposure which will be described later is provided on the upper surface of the lens holder. A light source 19 for reflection is provided at the side of the bellows 8 above the original. When the original 35 is transparent, for example, such as a film, a light source 38 provided below the original 35 is used.
The light emitted from the light source 19 is reflected by the original 35. The reflected light forms through the lens 11, an image of the original 35 on the film 37 . The flash lamp 14 is not used. An image is formed on the film 37 through the contact screen 36. The image has various sizes of dots corresponding to the tones of the contrast of the image of the original 35.
The flash exposure is for making contrast low between a shadow portion and a middle tone portion of an original. Referring to FIG. 4, in the flash exposure, the film 37 is uniformly exposed through the contact screen 36 by the flash lamp 14 provided on the lens holder of the bellows 8.
A bump exposure is for making contrast high between a highlight portion and a middle tone portion of an original. Referring to FIG. 5, the contact screen is removed from the original 37. The original 35 is illuminated by the light sources 19 or 38 and the reflected light or the transmitted light reaches the film 37.
Referring to FIG. 6, the flash exposure has the following effects. Suppose that a latent image shown by the curve 41 of FIG. 6 is formed through the main exposure. The flash exposure increases the exposure amount as shown by the broken curve 42. As a result, a part of the latent image reaches the integral exposure amount to form dots 43a and 43b. The dot 43b is a plan view of the formed dot. Only the dots of the middle to the lowest tone grow through the flash exposure. A shadow portion has a lower contrast and the screen range is enlarged.
Referring to FIG. 7, the bump exposure has the following effects. The exposure amount of the bump exposure is about 2-10% of the main exposure amount. In the bump exposure, as an original density is increased, a smaller amount of light is applied to a film. As a result, a middle tone portion and a highlight portion are exposed to a larger extent because the corresponding portions of the original have low densities. Therefore, the dot area of latent images 45a and 45b of the middle tone to the largest dot formed by the main exposure is increased, resulting in the dots having the larger area as latent images 44a and 44b. The bump exposure makes high the contrast in the highlight portion and reduces the screen range.
The bump exposure is in particular effective for reproducing precise tones of highlight regions and for eliminating dots from a white portion as in a case of an illustration drawn on a piece of white paper (a highlight process).
As the foregoing, in a sub-exposure method, a main exposure and one or both of a bump exposure and a flash exposure are properly combined, thereby obtaining a halftone negative image reproducing contrast of an original with fidelity.
As a conventional exposure control technique for implementing a sub-exposure method, for example, one is proposed by the present assignee and disclosed in Printing Magazine (1975, No. 8, Vol. 58, pp. 43-47) and another is disclosed in 35 years of Image Techniques (published on Oct. 11, 1978 pp. 93-99) published by the present assignee.
Referring to FIG. 9, a conventional exposure control apparatus includes a halftone data input unit 121 capable of inputting and setting basic data and original data.
The basic data includes basic data for main exposure, basic data for bump exposure and basic data for flash exposure.
Referring to FIGS. 8 and 9, for example, the basic data for main exposure includes a basic main exposure time T.sub.BM for reproducing a predetermined reference original in a desired dot percentage, and densities B.sub.MIN and B.sub.MAX corresponding to a dot percentage A.sub.H of a highlight region of the reference original and a dot percentage A.sub.S of a shadow portion, respectively when in halftone photography for the basic main exposure time T.sub.BM. In general, 95% and 5% are considered desired values for A.sub.H and A.sub.S, respectively.
The dot percentage represents a ratio of a halftone dot area to a unit area of an image formed on a film. If a screen range of a contact screen is B.sub.DR, the equation B.sub.DR =B.sub.MAX -B.sub.MIN is established.
According to the tone characteristic curve 46 of the main exposure (T.sub.BM) in FIG. 8, the reference original can be reproduced in the density range of B.sub.MIN -B.sub.MAX. As indicative by the curve 48, the maximum value of the density of the reproducible reference original is increased to B'.sub.MAX through the flash exposure (T.sub.BF). On the other hand as indicated by the curve 47, the minimum value of the density of the reproduced reference original is increased to B'.sub.MIN.
The basic data for a bump exposure includes a basic bump exposure time T.sub.BH and a reference original density B'.sub.MIN corresponding to an obtained dot percentage A.sub.H of a highlight region when subjected to bump exposure for the basic bump exposure time T.sub.BH after the halftone photography for the basic main exposure time T.sub.BM.
The basic data for flash exposure includes a basic flash exposure time T.sub.BF and a reference original density B'.sub.MAX corresponding to an obtained dot percentage A.sub.S of a shadow region when subjected to flash exposure for the basic flash exposure time T.sub.BF after the halftone photography for the basic main exposure time T.sub.BM.
The original data includes a highlight area density D.sub.MIN and a shadow area density D.sub.MAX from which a dot percentage of a highlight area A.sub.H and a dot percentage of a shadow area A.sub.S respectively, are obtained as a result of the halftone photography. A difference (D.sub.MAX -D.sub.MIN) between the shadow area density D.sub.MAX and the highlight area density D.sub.MIN denotes an original range D.sub.DR.
The conventional exposure control apparatus further includes an exposure control device 125 containing an exposure operating device 127 for calculating a main exposure amount T.sub.M, a bump exposure amount T.sub.H and a flash exposure amount T.sub.F based on the basic data and the original data to control exposure of the process camera.
The process camera includes an autofocus control circuit 30 and an automatic focusing device 32 for automatically adjusting focus based on set magnification m. The autofocus control circuit 30 includes an autofocus operating device 31. The magnification m is applied from the autofocus control circuit 30 to an exposure operating device 127. An f-number F.sub.NO and basic data T.sub.0 for a line exposure are also accepted by the exposure operating device 127 to carry out an exposure operation.
The conventional exposure control apparatus operates as follows. The basic data and the original data are applied to the halftone data input unit 121. The exposure operating device 127 obtains the main exposure time T.sub.M, and one or both of the flash exposure time T.sub.F and the bump exposure time T.sub.H through the operations based on the above-described equations (3)-(6). A prescribed exposure control is carried out based on the obtained values T.sub.M, T.sub.F and T.sub.H.
The operations of the values T.sub.M, T.sub.F and T.sub.H are separately performed in the following two cases depending on a relation between the original range and the screen range.
(I) When the original range is larger than the screen range (D.sub.DR &gt;B.sub.DR): EQU T.sub.M =T.sub.BM .multidot.10.sup.A . . . (3)
wherein A=D.sub.MIN -B.sub.MIN. EQU T.sub.F =T.sub.BF .multidot.(1-10.sup.-C)/(1-10.sup.-F) . . . (4)
wherein C=D.sub.Dr -B.sub.DR, and F=B'.sub.MAX -B.sub.MAX.
(II) When the original range is smaller than the screen range (D.sub.DR &gt;B.sub.DR): EQU T.sub.M =T.sub.BM .multidot.10.sup.B. . . (5)
wherein B=D.sub.MAX -B.sub.MAX. EQU T.sub.H =T.sub.BH .multidot.10.sup.D .multidot.(1-10.sup.C)/(1-10.sup.-E) . . . (6)
wherein D=D.sub.MIN -B'.sub.MIN and E=B'.sub.MIN -B.sub.MIN.
In actuality, corrections corresponding to the magnification m and a change of the common f-number F.sub.NO are added to these equations.
In general, the above-described conventional exposure control system has theoretically no problem provided that the exposure and the development can be strictly controlled. In actuality, however, quality of photosensitive material varies in sensitivity depending on a production lot. Also in an automatic development, activity of developer used therein varies to some extent. Furthermore, it is sometimes difficult to accurately measure a highlight area density or a shadow area density of the original data, which might result in that desired dot percentages A.sub.H and A.sub.S cannot be obtained of the portions corresponding to the highlight region and the shadow region of the halftone negative image obtained based on the basic data and original data.
In the above-described conventional method, the halftone photography is repeated after correcting the value D.sub.MIN or D.sub.MAX of the original in such a case. It is necessary to estimate the most suitable values of the original data D.sub.MIN and D.sub.MAX based on the result of the previous halftone photography. However, this estimation is so difficult that only an operator of considerable experience can make it accurately. In addition, a conventional apparatus requires the original data to be corrected in different manners depending on whether a photosensitive material for use is of a negative type or a positive type.